As suspected, iPhone OS 2.0 -- which was included with the Aspen Emulator in iPhone SDK beta 2 -- has been issued to approved iPhone developers, and subsequently leaked as an IPSW file that can be installed on physical devices.
The new release, like iPhone OS 1.2, is highly unstable and requires the use of "pwnage," a tool that leverages an exploit found in the lower levels of the iPhone/iPod Touch bootloader. Exercising the exploit allows these devices to run unsigned code. With this capability in place, Pwnage can modify the iPhone OS 1.2 or 2.0 IPSW (installation file), pre-activating and pre-jailbreaking it.
New features in the iPhone OS 2.0 release are sparse. Most significantly, there is now a "Contacts" application. Traditionally, contacts have been accessible only through the Phone application.
If you happen to come across a leaked copy of iPhone OS 2.0 we strongly recommend that you not attempt installation. Not only is the release unstable, causing potential problems with playback of iTunes content and more, but it is also illegal to posses. Installing the update will certainly void your warranty, and could potentiate legal action on Apple's part.